A lot of activities have been undertaken recently to develop optical pickups that are compatible with, for example, the BD (Blu-ray disc) and the HD-DVD (High Definition-DVD). A blue laser is used in the BD/HD-DVD compatible optical pickup to write or read information on the optical disc. Resin optical components in the optical pickup therefore need to be desired in view of optical resistance against blue light.
Light in the blue region have short wavelengths and very large light energy. In an objective lens made up of a resin base material and a high-refractive-index, anti-reflection film formed on the surface of the base material, the anti-reflection film may absorb light energy at the blue wavelengths and produce heat locally. The heat could reach the base material, deforming the surface of the base material. The heat conducted to the base material would also reduce resistance of the anti-reflection film against abrasion due to different coefficients of expansion of the anti-reflection film and the base material. Meanwhile, without the anti-reflection film being provided on the surface of the base material, the lens shows poor light transmittance and is hence less useful as an optical element. In addition, the surface will more likely degrade in oxidation.
Accordingly, for example, Japanese Unexamined Patent Publication No. 2005-317186 (Tokukai 2005-317186; published Nov. 10, 2005) teaches a technique of restraining deformation of the base material surface without reducing the light transmittance by providing an anti-reflection film having a refractive index of 1.7 or less on the base material surface. The anti-reflection film has a medium to low refractive index and produces less heat under radiation in the ultraviolet region than an anti-reflection film containing a layer with a high refractive index. The anti-reflection film thus restrains deformation of the base material surface without reducing the light transmittance even under high-optical-energy, short-wavelength light, including blue light.
BD/HD-DVD compatible optical pickups are often designed to operate with conventional DVDs and CDs. In that case, two objective lens are needed: one for blue light and the other for red and infrared light. The blue light objective lens, since designed for blue light, is made of a material which itself has a low absorption rate for blue light. The provision of the anti-reflection film of Tokukai 2005-317186 is effective.
On the other hand, however, conventional objective lenses for red and infrared light are made of olefin-based or alicyclic olefin-based resins. These materials themselves have a high absorption rate for blue light. No special consideration is paid to the anti-reflection film formed on the surface of those lenses as in Tokukai 2005-317186.The film degrades by absorbing light energy even from a small amount of blue light.
Table 1 shows data for various BD-R/RE writing schemes. Table 1 indicates that the net amount of blue light projected onto a blue light objective lens is 9.55 Wh/mm2 for SL×1 writing of a BD-RE. Meanwhile, for example, an objective lens made of resin for red and infrared light develops a defect when the net amount of light is in excess of 0.25 Wh/mm2. The figures predict that if about 3% of the blue light aimed at the blue light objective lens spills into the red and infrared light objective lens, it would be sufficient to degrade the red and infrared light objective lens.
TABLE 1NetEffectiveNetAmountAreaAmountofWritingof ObjectiveRadiationofRadiationBD-R/REPowerLensRadiationPulseRadiation(0.25 Wh/mm2Writing(mW)(mm2)Hours (h)Duty(Wh/mm2)Ratio)DL × 2143.1410,00050%22.281.1%DL × 1123.1410,00050%19.101.3%SL × 273.1410,00050%11.142.2%SL × 163.1410,00050%9.552.6%SL: Single Layer DiscDL: Double Layer Disc×1: Standard Speed×2: Double Speed
In the case of DL×2 writing, if only about 1% of the blue light spills into the red and infrared light objective lens, it would be sufficient to degrade the red and infrared light objective lens. So, far more stringent specifications are required for writing at double or even faster speed. Therefore, the blue light needs to be somehow prevented from spilling into the red and infrared light objective lens if a conventional red and infrared light objective lens is used in a BD/HD-DVD compatible optical pickup. It is difficult to form a single surface which transmits or reflects a few percent or even less of incoming blue light and also costly to fabricate such a film surface.